The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventor, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
A typical modern vehicle can employ a variable reluctance (VR) sensor to measure speed and/or position of a rotating shaft. Typical applications include a crankshaft position sensor, a transmission speed sensor, a wheel speed sensor, and the like. The VR sensor generates a sinusoidal voltage that increases in frequency and voltage as its associated shaft increases speed. A wave shaping circuit converts the sinusoidal signal to a square wave signal that is compatible with digital electronics such as a microcontroller.
A typical wave shaping circuit includes hysteresis to prevent its output from oscillating when the VR signal voltage changes relatively slowly, e.g. when the VR sensor is measuring a shaft that is rotating slowly. Since the magnitude of the VR signal voltage changes as a function of the shaft speed, it can be necessary to correspondingly change the hysteresis voltage.
Referring now to FIG. 1, a block diagram is shown of a VR sensor circuit in accordance with the prior art. A VR sensor 10 generates a sinusoidal output signal based on the speed of a rotating shaft 12. A VR sensor interface module 14 converts the sinusoidal signal to a digital signal that is compatible with a microcontroller 16. An example of VR sensor interface module 14 includes the MAX9924 device that is manufactured by MAXIM Integrated Products, Inc. Microcontroller 16 estimates the speed of rotating shaft 12 based on the digital signal. Microcontroller 16 also generates a pulse width modulation (PWM) signal 18 based on the shaft speed. The duty cycle of the PWM signal represents a desired hysteresis voltage that VR sensor interface module 14 should use while converting the sinusoidal signal. A low pass filter module 20 generates a bias signal 22 based on PWM signal 18. Bias signal 22 is applied to VR sensor interface module 14 to control its hysteresis voltage. A shortcoming with this design is that low pass filter module 20 limits how fast bias signal 22 can be changed. This can be a problem when rotating shaft 12 quickly changes speed and bias signal 22 materially lags the change, such as lags enough that noise enters the system and microcontroller 16 misinterprets the shaft speed.